A number of immunochemical methods have so far been used for measurement of physiologically active substances contained in body fluids such blood, urine and the like. Among them, a measuring reagent using finely divided granular solids such as blood cells, high molecular latexes, etc. as a carrier for the antigen or antibody on the basis of the immunochemical agglutination reaction or agglutination inhibition reaction has found wide applications in the measurement of the abovementioned physiologically active substances because it can be easily employed and rapidly provides the results of measurement.
The reagent is used, for example, for the diagonis of pregnancy by way of measurement of gonadotropin (hCG) contained in the serum or urine of a woman who might be pregnant, or for the diagnosis of growth of an fetus by measuring esteriol in the urine of a pregnant woman.
Accordingly, the essential requirements for the reagent are as follows. First, the reagent must have such a high sensitivity of measurement that when even a trace amount of a matter to be detected is present, the reagent causes the agglutination reaction or the agglutination inhibition reaction on the basis of the immunochemical reaction occurring between the matter to be detected and the reagent, and thus enables the measurement of the matter to be detected. Second, the reagent must have such a high specificity that unless the matter to be detected is present, the reagent does not cause the agglutination reaction or the agglutination inhibition reaction. Furthermore, the reagent must maintain the sensitivity and the specificity as they are immediately after the production of the reagent even after the reagent is stored for an extended period of time. That is to say, the reagent must have good storability.
It has heretofore been difficult for a reagent using finely divided solid particles to keep its sensitivity of measurement and specificity as they are immediately after the production for a long period of time. Hence, the reagent has the disadvantages that it has a short effective period or frequently provides a measurement error even during the effective period.
To cope with these problems, there have conventionally been proposed various methods including, for example, a method of covering in advance the finely divided solid particles with a protein that does not participate in the immunoreaction and then letting the antigen or the antibody to be absorbed onto the finely divided solid particles, or a method of first letting the antigen or the antibody to be adsorbed onto the finely divided solid particles and then treating them with a protein solution that does not participate in the immuno-reaction (Japanese Patent Publication No. 12,471/1968, Japanese Patent Publication No. 11,407/1974, Japanese Patent Laid-Open Publication No. 82,230/1975).
These prior art methods use, as the proteins that do not participate in the immuno-reaction, blood serum albumin (BSA), egg albumin (EA), lactoalbumin, hemoglobin, blood serum globulin, lactoglobulin, and the like. Though these proteins provide certain effects, their effects are not entirely satisfactory.
The inventors of the present invention has made intensive studies in search for a stabilizer which enables a reagent to sufficiently maintain its high measuring sensitivity and specificity over an extended period of time, and now found that cytocrome c provides outstanding effect. The present invention is completed on the basis of this finding.